Various methods have been proposed for printing half-tone images. The following are background examples: U.S. Pat. No. 4,084,259, EP-A-126 782 and EP-A-201 674. See also, U.S. Pat. No. 3,197,558 and RCA Review, Vol. 31, No. 3 (9/70) pp. 517-533 (Electronically Dithered Half-Tone Pictures). Improvements in reproduction in raster form have been developed which substantially enhance half-tone image quality. Generally, such a system utilizes image elements according to a raster pattern of image points. Dots are printed in the image points of each element with intensities based upon intensity data contained in the image information.
An improved method is described in EP-A-190 901 in which the optical density scale of the reproduction system is divided into a number of zones. Each zone is allocated a pattern of N contiguous image points on a raster pattern of m .times. n image points, where N &lt; (m .times. n). For each zone, a different number N is selected. During the actual printing, dots (that is black spots) are printed on the image support in the N image points only. A number of intensity levels within a zone are attained for each pattern of image points by allocating different distributions of intensity values to the N image points of the pattern. The dimensions of the dots printed in the image points vary with the intensity value allocated to each image point; that is, the dimensions increase with increasing intensity value to provide a print having a darker appearance. For high intensity values, the printed dots increase beyond the raster boundaries of their image points and partially overlap adjacent image points. In this way each combination of pattern and distribution of intensity values corresponds to a value in the optical density scale. This combination is stored in a memory of a raster pattern generator.
On the basis of the intensity data fixed in the image information the raster pattern generator calls up from its memory the corresponding data concerning the pattern of image points wherein a dot is to be printed and the intensity values applicable per image point and transmits these data to the control unit of a printer in order to provide the image reproduction.
In this method, care is taken to select patterns of image points wherein a dot is to be printed, that take the form of clusters having a large perimeter to obtain a wide dynamic range of gradation. Preferably, the patterns are L-shaped clusters or combinations thereof.
When the known method is used in electrographic laser printers problems arise when the photoconduction properties of the intermediate image support change over time and become uneven over one print area. In laser printers a photoconductive intermediate image support is evenly charged and exposed to a laser beam modulated in accordance with the image information. The image is developed with toner powder using a bias voltage to deposit toner powder onto the exposed areas. The laser beam normally has a more or less circular cross-section and the intensity distribution over the diameter of the beam generally has a Gaussian form; however, in the center of the laser beam, the light intensity may be enough to fully discharge the intermediate image support. Surrounding the center of the beam is an area of lower energy which only partially discharges the intermediate image support. In this lower energy area, system irregularities become apparent which tend to seriously decrease the overall print quality.
Accordingly, it is an object of the present invention to provide a method and apparatus which deliver prints with a consistent image quality.